Chilled water skid for natural gas processing

ABSTRACT

An improved chilled water skid for natural gas processing is provided. The skid includes an apparatus for out a refrigeration cycle. The apparatus includes an evaporator, a compressor, a condenser, a refrigerant reservoir and a control valve positioned downstream of the compressor and upstream of the refrigerant reservoir. The control valve is configured to regulate a pressure on a downstream side of the compressor.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. provisional application Ser. No. 61/176,170, filed May 7, 2009, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present application relates to natural gas processing. In particular, the present application relates to the stripping of entrained liquids from natural gas at low ambient temperatures.

Much of the natural gas produced in the U.S. and offshore is referred to as “wet” gas because of entrained liquids such as butane, isobutane, propane, isopropane, ethane, natural gasoline and other liquids. Gas transmission pipelines have requirements for the type of gas they will accept and one of the important requirements is dry gas having a standard btu content. Therefore, it has been standard practice in the U.S. to build conditioning plants near new natural gas fields to condition the gas from the wellhead, before it enters a transmission pipeline. Natural gas is drawn from a wellhead at temperatures that typically range from 130° F. to 150° F. The hot natural gas generally includes hydrocarbon liquids and other entrained liquids. Because the presence of entrained liquid in a pipeline can cause equipment damage and/or failure, the entrained liquids must be removed from the natural gas before the gas is introduced to the pipeline for distribution.

One common way to condition the gas or remove the impurities and entrained liquids is to lower the temperature of the gas exiting the well head and strip the entrained liquids. Because the entrained liquids have lower boiling points than the desirable components of natural gas, the liquids can be easily separated from the chilled gas. Economic pressures, however affect the operating decisions of the well heads and the stripping systems. For example, there may be a break even price for natural gas below which it is not profitable to operate a particular well. In other situations, as a well is used, the production rate may go down which results in a greater cost per cubic foot of gas produced. In these situations, it is advisable to have semi-portable skids for providing the stripping and other process equipment near a wellhead such that the equipment may be easily redeployed if desired.

In many situations, at least two skids will be deployed. First, a process skid that will include the equipment for stripping the gas stream including a vapor-liquid separator vessel, which also accumulates the stripped liquids, and heat exchangers for cooling the hot gas. Second, a chilled water skid may be used. The chilled water skid provides cool water (or a water/antifreeze mixture) for use with the heat exchangers of the process skid to strip the hot gas. The chilled water skid operates a refrigeration cycle to provide chilled water/antifreeze. The chilled water skid has generally relied on a chiller using propane as the refrigerant. For example, U.S. Pat. No. 5,687,584 issued to Mehra discloses the use of propane base refrigeration equipment for conditioning natural gas. U.S. Pat. No. 3,531,943 issued to Pamag et al., discloses the use of hydrocarbon refrigerants including propane and methane. However, there are safety issues associated with using flammable hydrocarbons as a refrigerant, especially while trying to minimize maintenance and staffing costs.

Conventional HVAC type Freon refrigeration chiller systems are designed to operate at evaporator temperatures to produce a moderately cool water for utility use, and in relatively warm ambient conditions. When exposed to low ambient temperature operating conditions, conventional design Freon refrigeration systems cannot start, or continue to operate due to low pressure suction conditions to the Freon compressor. Such systems may include a programmable logic controller (PLC) configured to prevent restart of the compressor is a certain minimum downstream pressure or pressure differential is not measured. The PLC is programmed with a control logic that makes decisions regarding restarts, valve settings, and other system operations based upon measured parameters such as pressures and temperatures. The systems are normally designed to protect the compressors through control schemes that detect this condition and prevent start or re-start of the system.

It is desirable to operate the skids without human technician intervention, and particularly without on-site human intervention. However, the operation of a refrigeration cycle depends on the maintenance of certain minimum operating conditions including pressure increases across compressors and other requirements. In lower ambient temperatures, such as those found during winter and at higher altitudes, it may be difficult to maintain these conditions which can result in shut downs of the skids that won't permit restarts without human intervention.

Accordingly, there is a need for a system to maintain ongoing well head operations for a natural gas well while minimizing the need for human intervention. There is a further need for a system to maintain ongoing well head operations for a natural gas well that does not utilize propane as a refrigerant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a chilled water skid according to the prior art.

FIG. 2 is a schematic view of a chilled water skid.

FIG. 3 is a schematic view of a chilled water skid.

DETAILED DESCRIPTION

Referring to FIG. 1, a prior art chilled water skid includes a refrigeration loop 10, having an evaporator 12, a compressor 14, and a condenser 16. Stream 18 provides hot coolant (i.e. water or a water/antifreeze mix) to evaporator 12 where the coolant is chilled to provide a cold stream 20. Stream 20 is provided to a process skid for stripping the well gas.

A refrigerant is circulated through loop 10. The refrigerant may be any suitable refrigerant such as Freon or other suitable component. Stream 22 carries evaporated refrigerant away from evaporator 12 and to compressor 14. Compressor 14 pressurizes the evaporated refrigerant. The temperature of the refrigerant may be higher on the downstream side of compressor 14 than it is on the upstream side. Stream 24 carries the pressurized refrigerant to condenser 16. The warm pressurized refrigerant is cooled to a condensed liquid. Stream 26 may carry the liquid refrigerant to evaporator 12. A pump may be used to circulate the refrigerant through loop 10. The pump may be positioned on stream 26.

Systems such as those shown in FIG. 1 may include process control elements that shut down the refrigeration loop in the event that minimum temperature drop is not achieved between streams 18 and 20. This may occur in low ambient temperature conditions where the coolant is chilled by the air as the coolant is moved from the process skid to the chilled water skid. The system may then be configured to restart when the accumulated heat in the coolant results in a sufficient temperature in stream 18 to warrant restarting the refrigeration loop 10. At restart, a control logic may base a restart decision, at least in part, on the differential pressure across the compressor 14. The control logic may be programmed to make a restart decision based on a measured pressure at a point downstream of the condenser or it may be based on the differential pressure across the condenser. If there is not a sufficient differential pressure, the compressor could be damaged upon restart. This can lead to the situation where the refrigeration cycle needs to be run to cool the coolant stream, but the compressor is unable to restart automatically. As a result, a human technician may be required to manually restart the compressor.

Referring to FIG. 2, a refrigeration loop 110 may include an evaporator 112, a compressor 114, a condenser 116, and a refrigerant reservoir 136. Stream 118 provides hot coolant (i.e. water or a water/antifreeze mix) to evaporator 112 where the coolant is chilled to provide a cold stream 120. Stream 120 is provided to a process skid for stripping the well gas.

A refrigerant is circulated through loop 110. The refrigerant may be any suitable refrigerant such as Freon, or other suitable component. Stream 122 carries evaporated refrigerant away from evaporator 112 and to compressor 114. Compressor 114 pressurizes the evaporated refrigerant. The temperature of the refrigerant may be higher on the downstream side of compressor 114 than it is on the upstream side. Stream 124 carries the pressurized refrigerant to condenser 116. A second portion of the hot compressed refrigerant gas from compressor may be passed through stream 132 to reservoir 136. A pressure control valve 134 may be used to regulate the pressure on the downstream side of compressor 114 to maintain a minimum differential pressure across compressor 114 to allow for cold weather restarts. The warm pressurized refrigerant is cooled to a condensed liquid. Stream 126 may carry the liquid refrigerant to reservoir 136. Stream 128 may be provided to deliver uncondensed liquid refrigerant from condenser 116 to reservoir 136. A pressure control valve 130 may be used to regulate the differential pressure across condenser 116. Stream 138 may be passed through an expansion valve 140 prior to being provided to evaporator 112. A pump may be used to circulate the refrigerant through loop 110. The inclusion of reservoir 136 allows for ample refrigerant to be included in loop 110 to allow for the regulation of differential pressures across components. Optional manual valves may also be included for use by human operators.

A control logic controls the control valves 130 and 134 to maintain adequate differential pressures across the refrigeration loop components for automatic restart. For example, a control valve may be opened or closed based on the downstream side pressure from compressor 114. Alternatively, the valve may be controlled based on the differential pressure across compressor 114 as measured by pressure sensors on the upstream and downstream sides of compressor 114. This allows the refrigeration loop to be cycled on and off during cool weather thereby conserving energy and lowering operating expenses, without the need for a human technician to be on-site for system restarts. The system also allows for the use of Freon or other refrigerants as prior chilled water skids have relied on propane based refrigeration cycles. A second independent refrigerant loop may also be used for redundancy.

Referring to FIG. 3, a chilled water skid may include a first refrigerant loop 210 and a second redundant refrigerant loop 211. Corresponding components of loop 210 and 211 are designated as ‘a’ or ‘b’ respectively, and reference will be made to the components of loop 210. Refrigeration loop 210 may include an evaporator 212, a compressor 214, a condenser 216, and a refrigerant reservoir 236. Stream 218 provides hot coolant (i.e. water or a water/antifreeze mix) to evaporator 212 where the coolant is chilled to provide a cold stream 220. Stream 220 is provided to a process skid for stripping the well gas.

A refrigerant is circulated through loop 210. The refrigerant may be any suitable refrigerant such as Freon, or other suitable component. Stream 222 carries evaporated refrigerant away from evaporator 212 and to compressor 214. Compressor 214 pressurizes the evaporated refrigerant. The temperature of the refrigerant may be higher on the downstream side of compressor 214 than it is on the upstream side. Stream 224 carries the pressurized refrigerant to condenser 216. A pressure control valve 234 may be used to regulate the pressure on the downstream side of compressor 214 to maintain a minimum differential pressure across compressor 214 to allow for cold weather restarts. The warm pressurized refrigerant is cooled to a condensed liquid. Stream 226 may carry the refrigerant to reservoir 236. Stream 238 may be passed through an expansion valve 240 prior to being provided to evaporator 212. A pump may be used to circulate the refrigerant through loop 210. The inclusion of reservoir 236 allows for ample refrigerant to be included in loop 210 to allow for the regulation of differential pressures across components. Optional manual valves 242 may also be included for use by human operators.

A control logic controls the control valves to maintain adequate differential pressures across the refrigeration loop components for automatic restart. This allows the refrigeration loop to be cycled on and off during cool weather thereby conserving energy and lowering operating expenses, without the need for a human technician to be on-site for system restarts. The system also allows for the use of Freon or other refrigerants as prior chilled water skids have relied on propane based refrigeration cycles.

In some embodiments, off the shelf chillers available from Carrier CORP. of Farmington, Conn., may be used as the basis for a chilled water skid including model numbers: 30 GXN, 30 GTN, 30 RAN, 30 GXR, 30 GX, 30 GT and 30 XAA. Use of off the shelf chillers saves on manufacturing costs. Many prior art chillers used custom designed equipment. The aforementioned Carrier units may be modified by the addition of one or more backpressure valves installed on the inlet of each of two or more refrigerant condensors. The valves are activated by sensing compressor discharge pressure and release pressure as needed to maintain both a minimum required pressure and protect from overpressure. In one such embodiment, the following operating parameters may be observed:

Coolant temperature leaving chilled water +14 degrees F. skid to process (LWT): Freon temperature in chiller barrel: +4 degrees F. Compressor suction pressure: 5-10 psig Compressor discharge pressure: 175 psig Ambient temperature: minus 15 degrees F.

To ensure cold weather restart, the differential pressure across the compressor must be at least about 50 psi.

In other embodiments, an off the shelf chiller available from York of York, Pa., may be used as the basis for a chilled water skid, model number YCAS. Use of off the shelf chillers saves on manufacturing costs. Many prior art chillers used custom designed equipment. The YORK chiller may be modified by the addition of a refrigeration reservoir and a regulated condenser bypass. In one such embodiment, the following operating parameters may be observed:

Coolant temperature leaving chilled water zero degrees F. skid to process (LWT): Freon temperature in chiller barrel: minus 10 degrees F. Compressor suction pressure: 5-10 psig Compressor discharge pressure: 175 psig Ambient temperature: minus 15 degrees F.

To ensure cold weather restart, the differential pressure across the compressor must be at least about 50 psi. 

1. An apparatus for carrying out a refrigeration cycle, the apparatus comprising: an evaporator; a compressor; a condenser; a refrigerant reservoir; and a control valve positioned downstream of the compressor and upstream of the refrigerant reservoir wherein the control valve is configured to regulate a pressure on a downstream side of the compressor.
 2. The apparatus of claim 1, wherein the condenser is positioned downstream of the control valve and upstream of the refrigerant reservoir.
 3. The apparatus of claim 1, further comprising a second control valve positioned downstream of the condenser and upstream of the refrigerant reservoir.
 4. The apparatus of claim 1, wherein the control valve is configured to regulate the downstream side pressure of the compressor based on a measured pressure on the downstream side of the compressor.
 5. The apparatus of claim 4, wherein the control valve is configured to maintain a pressure of at least about 60 psig at a point downstream of the compressor.
 6. The apparatus of claim 1, wherein the control valve is configured to regulate the downstream side pressure of the compressor based on a measured differential pressure across the compressor.
 7. The apparatus of claim 6, wherein the control valve is configured to maintain a pressure differential of at least about 50 psi across the compressor. 